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

Highly transmissive, superconducting contacts of mum-scale have been m ade to the two-dimensional electron ps in GaAs/AlGaAs heterostructures , in combination with lateral gate structures. We present results of r esistance measurements at T almost-equal-to 10 mK on a sample with a q uantum point contact between two superconducting Sn/Ti contacts. New e ffects due to phase-coherent Andreev reflection have been observed. Th e conductance is found to be quantized with enhanced steps > 2e2/h, wh ich is attributed to Andreev reflection. At zero gate voltage an enhan cement of weak localization by Andreev reflection has been measured fo r the first time, while at a higher gate voltage indications of an exc ess conductance due to reflectionless tunneling have been found.