  We report the occurrence of the zero-bias conductance peak (ZBCP) in an InAs
nanowire coupled to PbIn superconductors with varying temperature, bias
voltage, and magnetic field. The ZBCP is suppressed with increasing temperature
and bias voltage above the Thouless energy of the nanowire. Applying a magnetic
field also diminishes the ZBCP when the resultant magnetic flux reaches the
magnetic flux quantum h/2e. Our observations are consistent with theoretical
expectations of reflectionless tunneling, in which the phase coherence between
an electron and its Andreev-reflected hole induces the ZBCP as long as
time-reversal symmetry is preserved.
