  We investigate the proximity effect in InAs nanowire (NW) junctions with
superconducting contacts made of Al. The carrier density in InAs is tuned by
means of the back gate voltage $V_g$. At high positive $V_g$ the devices
feature transport signatures characteristic of diffusive junctions with highly
transparent interfaces - sizable excess current, re-entrant resistance effect
and proximity gap values ($\Delta_N$) close to the Al gap ($\Delta_0$). At
decreasing $V_g$, we observe a reduction of the proximity gap down to
$\Delta_N\approx\Delta_0/2$ at NW conductances $\sim2e^2/h$, which is
interpreted in terms of carrier density dependent reduction of the Al/InAs
interface transparency. We demonstrate that the experimental behavior of
$\Delta_N$ is closely reproduced by a model with shallow potential barrier at
the Al/InAs interface.
