Gate-modified giant Andreev backscattering

We investigated giant Andreev backscattering in a superconductor (S) - two- dimensional electron gas (2DEG) junction with two kinds of gate: a quantum point contact (QPC) and an ordinary wide gate. Though the diffusive 2DEG it self displayed reentrant conductance, by attaching a QPC we observed conduc tance enhancement (peak) at zero energy. The bias voltage and the magnetic field dependence of the enhancement at different degrees of dirtiness showe d that phase conjugation between an electron and a hole is essential for en hancement. (C) 2000 Elsevier Science B.V. All rights reserved.