All signal propagation in circuits or through free space is finally happening as electromagnetic waves. The signal is NOT in the metal of wires, its in the electromagnetic field around the wires. The conductive wires only force the waves to travel to wanted directions.
Current in metal is a measurable side product that the EM field causes. Voltage is a scalar quantity which presents EM field in radically simplified way. Voltage and current together tell practically enough of the EM field at low enough frequencies and in a small enough circuit where one can succesfully assume that no waves exist and everything happens in the whole circuit at the same time.
But how high the frequency can be and how large can circuit dimensions be when one wants that ignoring waves doesn't cause too much error? No strict limit exists. But I would take propagating as waves into the account if the circuit width or length is 10% of the free space wavelength of the frequency.
If the frequency is 100MHz the wavelength in free space is 3 meters. That means I shouldn't ignore waves in a circuit which has biggest dimension 30 centimeters. Ignoring would cause so much error that nothing works.
The same experience has proved for me in practice that if the frequency is 100MHz and the biggest dimension of the circuit is only 3 centimeters (=1% of wavelength) I can well assume the signal propagates through the circuit in zero time and calculate the function of the circuit pretty well with Ohm's and Kirchoff's laws without substantial error. But the waves still exist. One can very likely detect with a radio receiver the small part of signal energy which jumps out of an unshielded 100MHz circuit (=radiates), no matter the biggest dimension of the circuit is only 3 cm. The 3cm size only makes small the error (in currents and voltages) caused by ignoring waves.
To make my 100 Mhz and 30 cm long circuit in perfectly predictable way I must design the wiring as transmission lines and take into the account how much phase lag is caused by the propagation time through the transmission line.
The simplest transmission lines are those which have 2 parallel wires. Twisted pair, coaxial cable and microstrip line belong to that category.
One wire transmission line exists. It's the Goubau-line. It needs not another wire.
https://en.wikipedia.org/wiki/Goubau_line
About reflections at 0.1Hz
There's an older answer which says it. You cannot have long enough distance to be able to measure the phase lag caused to 0.1Hz signal. In addition to generate and receive 0.1Hz practically detectable waves in free space with achievable voltages one would need say 300000 kilometres long antenna. Jupiter planet or the sun would be big enough reflectors to generate echos which have detectable power.
