  We have investigated experimentally the electronic transport properties of a
two-dimensional electron gas (2DEG) present in an AlSb/InAs/AlSb quantum well,
where part of the toplayer has been replaced by a superconducting Nb strip,
with an energy gap $\Delta_0$. By measuring the lateral electronic transport
underneath the superconductor, and comparing the experimental results with a
model based on the Bogoliubov-de Gennes equation and the Landauer-B\"uttiker
formalism, we obtain a decay length $\xi_{\text{Sm}} \approx 100~\text{nm}$ for
electrons. This decay length corresponds to an interface transparency
$T_{\text{SIN}}=0.7$ between the Nb and InAs. Using this value, we infer an
energy gap in the excitation spectrum of the SQW of $\Delta_{\text{eff}} = 0.97
\Delta_0 = 0.83~\text{meV}$.

Condensed Matter