A superconductor/semiconductor/superconductor junction with a long-split gate

Measurements of superconducting and normal transport in a superconductor/se miconductor/superconductor junction with a long-split gate show that when t he two-dimensional electron gas in the semiconductor is put into the pinche d-off state by applying a gate voltage, the two superconducting electrodes couple through a long and narrow channel with a small number of modes. Mult iple Andreev reflections the focusing of Andreev-reflected holes are observ ed in this situation, but the supercurrent decreases quickly as gate voltag e is increased and it disappears when the channel is long and narrow. A sha rp conductance peak due to the coherent motion of the electrons and holes i n the narrow channel is also observed. (C) 1999 Academic Press.