  We report quantum interference effects in InAs semiconductor nanowires
strongly coupled to superconducting electrodes. In the normal state, universal
conductance fluctuations are investigated as a function of magnetic field,
temperature, bias and gate voltage. The results are found to be in good
agreement with theoretical predictions for weakly disordered one-dimensional
conductors. In the superconducting state, the fluctuation amplitude is enhanced
by a factor up to ~ 1.6, which is attributed to a doubling of charge transport
via Andreev reflection. At a temperature of 4.2 K, well above the Thouless
temperature, conductance fluctuations are almost entirely suppressed, and the
nanowire conductance exhibits anomalous quantization in steps of e^{2}/h.
