Conductance enhancement in quantum-point-contact semiconductor-superconductor devices

We present numerical calculations of the conductance of an interface betwee n a phase-coherent two-dimensional electron gas and a superconductor with a quantum point contact in the normal region. Using a scattering matrix appr oach we reconsider the geometry of De Raedt, Michielsen, and Klapwijk [Phys . Rev. B 50, 631 (1994)] which was studied within the time-dependent Bogoli ubov-de Gennes formalism. We find that the factor-of-2 enhancement of the c onductance G(NS) compared to the normal state conductance GN for ideal inte rfaces may be suppressed for interfaces with a quantum point contact with o nly a few propagating modes. The suppression is found to depend strongly on the position of the Fermi level. We also study the suppression due to a ba rrier at the interface and find an anomalous behavior caused by quasipartic le interference. Finally, we consider the limit of sequential tunneling and find a suppression of the factor-of-2 enhancement which may explain the ab sence of conductance enhanced in experiments on metal-superconductor struct ures. [S0163-1829(99)07943-6].