Scaling Theory of Two-Dimensional Metal-Insulator Transitions

We discuss the recently discovered two-dimensional metal-insulator transition in zero magnetic field in the light of the scaling theory of localization. We demonstrate that the observed symmetry relating conductivity and resistivity follows directly from the quantum critical behavior associated with such a transition. In addition, we show that very general scaling considerations imply that any disordered two-dimensional metal is a perfect metal, but most likely not a Fermi liquid.