  We show that the Wiedemann-Franz law is violated in the vortex state of a
d-wave superconductor at zero temperature. We use a semiclassical approach,
which includes the Doppler shift on the quasiparticles as well as the Andreev
scattering from a random distribution of vortices. We also show that the vertex
corrections to the electrical conductivity due to the anisotropy of impurity
scattering become unimportant in the presence of a sufficiently large magnetic
field.
