Electron transport in mesoscopic GaAs/AlGaAs-structures with superconducting contacts

For the first time an enhancement of weak localization by Andreev-reflection has been measured. This means that the interface between the Sn/Ti contacts and the two-dimensional electron gas (2DEG) in the GaAs/AlGaAs-heterostructure is practically barrier-free. The effect can be depressed by increased temperature, a DC bias current or a small magnetic field, but also by a voltage on a nearby gate. Also the sign of the temperature dependence of the differential resistance changes when the weak localization has disappeared. Further we report preliminary measurements on superconducting contacts to a shallow 2DEG. The transmission of this interface is probably not very high, since there are indications for an excess conductance caused by reflectionless tunneling. (orig.)