  Non-monotonic differential resistance (dV/dI) is observed in magnetically
induced insulating films which exhibit apparent superconductor-metal-insulator
transitions in the low temperature limit; at low bias currents the nonlinear
transport is insulator-like while at high bias currents it is characteristic of
metallic phase. The non-monotonic dV/dI may be evidence that the insulating
state consists of metallic domains connected by point contacts (insulating
gaps), implying that spatial inhomogeneities play a dominant role in
determining the nature of the apparent metal-insulator transition.
