Take a look at the PowerPath Controllers LTC4412 or LTC4417 from Linear Technology. They have some more of these PowerPath devices.

Take a look at the PowerPath Controller LTC4412 or the Prioritized PowerPath Controller LTC4417 from Linear Technology. They have some more of these PowerPath devices.

Or you can take a relay. The wall adapter controls the relay to open/close the line to the battery. AC wall adapter plugged in, relay on and battery line disconnected, vice versa. Then you have no voltage drop.

With the use of diodes, even shottky, you always have the disadvantage of the diodes voltage drop. And if the circuits current consumption is high, the size of the diodes will increase. The problem with voltage drop will get worse.