The diffusion tensor is obtained from 125 where D is the zero-field diffusion constant (1.14). One easily verifies that Dyy = Dxx and Dxy = −Dyx. From the Einstein relation one then obtains the conductivity tensor 126 with σ the zero-field conductivity (1.9). The resistivity tensor _ ≡ _−1 has the form 127 with ρ = σ−1 = m/nse2τ the zero-field resistivity.