  Semiconductor nanowires provide promising low-dimensional systems for the
study of quantum transport phenomena in combination with superconductivity.
Here we investigate the competition between the Coulomb blockade effect,
Andreev reflection, and quantum interference, in InAs and InP nanowires
connected to aluminum-based superconducting electrodes. We compare three
limiting cases depending on the tunnel coupling strength and the characteristic
Coulomb interaction energy. For weak coupling and large charging energy,
negative differential conductance is observed as a direct consequence of the
BCS density of states in the leads. For intermediate coupling and charging
energy smaller than the superconducting gap, the current-voltage characteristic
is dominated by Andreev reflection and Coulomb blockade produces an effect only
near zero bias. For almost ideal contact transparencies and negligible charging
energy, we observe universal conductance fluctuations whose amplitude is
enhanced due to Andreev reflection at the contacts.
