INFLUENCE OF GATE VOLTAGE ON THE TRANSPORT-PROPERTIES OF SUPERCONDUCTOR 2DEG SYSTEMS

Highly transmissive, superconducting contacts of mum-scale have been made to the two-dimensional electron ps in GaAs/AlGaAs heterostructures, in combination with lateral gate structures. We present results of resistance measurements at T almost-equal-to 10 mK on a sample with a quantum point contact between two superconducting Sn/Ti contacts. New effects due to phase-coherent Andreev reflection have been observed. The conductance is found to be quantized with enhanced steps > 2e2/h, which is attributed to Andreev reflection. At zero gate voltage an enhancement of weak localization by Andreev reflection has been measured for the first time, while at a higher gate voltage indications of an excess conductance due to reflectionless tunneling have been found.