Observation of a well defined transition from weak to strong localization in two dimensions

We have measured the low temperature T20 K magnetoconductance (MC) of ultrathin films that have strong spin-orbit interactions and conductances G spanning the weakly GG00=e2/22Latin small letter h with stroke and strongly localized GG00 regimes. For GG00, the MC is negative in agreement with weak localization theory, and for GG00, it is positive in accord with theories of electron hopping transport. The MC changes sign when GG00, independent of material and temperature. The data suggest that a well defined conductance separates the weak and strong localization regimes in two dimensions.