We discuss superconductor to insulator and quantum Hall transitions which are of first order in the clean limit. Disorder creates a nearly percolating network of the minority phase. Electrical transport is dominated by tunneling or activation through the saddle point junctions, whose typical resistance is calculated as a function of magnetic field. In the Boltzmann regime, this approach yields resistivity laws which agree with recent experiments in both classes of systems. We discuss the origin of dissipation at zero temperature.

• Received 17 October 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.3352