  We have probed the switching dynamics of the Josephson critical current of a
superconducting weak link by measuring its voltage/current characteristics
while applying an ac current bias in the range 1-200 MHz. The weak link between
two Nb reservoirs is formed by an mesoscopic Al wire above its critical
temperature. We observe a dynamical phase transition as a function of the
frequency and amplitude of the ac current. While at low frequency the
transition driven by increasing the current bias is well described by the
standard Kramers theory, at high frequency the switching histograms become
hysteretic and much narrower than expected by thermal fluctuations. The
crossover frequency between the two regimes is set by the electron-phonon
interaction rate in the normal metal.
