Edge states, transmission matrices, and the Hall resistance

We consider the Landauer formula, relating conductances to transmission matrices, for a two-dimensional system in a magnetic field. We argue that the magnetoresistance, R, and the Hall resistance, R₇, satisfy the sum rule (R+R₇) ^-1= (e^2/h) Tr (t^^t) where t is the transmission matrix. For zero field our expressions reduce to the usual multichannel Landauer formulas. In the absence of dissipation, R approaches zero, t approaches a unit matrix, and quantized values are obtained for the Hall resistance.