  The current-voltage I-V characteristics of a 1.2 \mum long Au nanowire
contacted by superconducting electrodes were studied in details. Interestingly,
the I-V curves over a wide range of temperatures display multiple steps at
voltages V = (m/n)(V0/2e) in the absence of microwave radiation, where m, n are
integer numbers and V0 ~ 92 \muV. We posit that these steps are the subharmonic
Shapiro steps due to the interplay of the ac Josephson current and a minigap in
the Au nanowire induced by superconducting electrodes via the proximity effect.
