  The transport through a metal-superconductor interface is governed by a
special charge conversion process, the Andreev reflection, where each incident
electron drags another electron with itself to form a Cooper pair. At the
normal side a hole is left behind dressed by superconducting correlations. For
a low transparency interface the simultaneous transfer of two charges is
strongly suppressed leading to a reduced conductance. Here we demonstrate that
this reduced conductance can be turned to an infinite one by tuning the
nanoscale geometry. Creating variable size nanojunctions between a thin
metallic film and a superconducting tip we study how multiple phase-coherent
scatterings enhance the superconducting correlations at the normal side. By
increasing the coherent volume of carriers initially the transmission through
the interface is continuously enhanced. However, as the phase-coherent volume
reaches the opposite surface of the thin film a resonator is formed, and a
robust transition is induced due to Cooper pair condensation.
