  Electron transport and tunneling across the superconductor-insulator (SI)
transition have been measured simultaneously for quench-condensed ultrathin
amorphous beryllium films. The anomalous negative magnetoresistance previously
observed in insulating films disappears when Mn impurities are introduced to
the films, restoring a rather clean Efros-Shklovskii type hopping behavior. The
combination of transport and tunneling data allows us to determine,
independently and up to a constant on the order of unity, the localization
length, \xi_{L}, and the dielectric constant, \kappa, for the films. As the
normal-state sheet resistance of the films at 20 K is reduced with increasing
film thickness, \xi_{L} increases exponentially. The SI transition occurs when
\xi_{L} crosses the Ginzburg-Landau coherence length, \xi_{S}.
