  The proximity effect in semiconductor-superconductor nanowires is expected to
generate an induced gap in the semiconductor. The magnitude of this induced
gap, together with the semiconductor properties like the spin-orbit coupling
and $g$\,-\,factor, depends on the coupling between the materials. It is
predicted that this coupling can be adjusted through the use of electric
fields. We study this phenomena in InSb/Al/Pt hybrids using nonlocal
spectroscopy. We show that these hybrids can be tuned such that the
semiconductor and superconductor are strongly coupled. In this case, the
induced gap is similar to the superconducting gap in the Al/Pt shell and closes
only at high magnetic fields. In contrast, the coupling can be suppressed which
leads to a strong reduction of the induced gap and critical magnetic field. At
the crossover between the strong-coupling and weak-coupling regimes, we observe
the closing and reopening of the induced gap in the bulk of a nanowire.
Contrary to expectations, it is not accompanied by the formation of zero-bias
peaks in the local conductance spectra. As a result, this cannot be attributed
conclusively to the anticipated topological phase transition and we discuss
possible alternative explanations.
