  We show that the Wiedemann-Franz law is violated at zero temperature in the
vortex state of a d-wave BCS superconductor with isotropic impurity scattering.
We use a semiclassical approach to include the Doppler shift experienced by the
quasiparticles due to the circulating supercurrents and consider as well the
Andreev scattering from an array of vortices assumed to be randomly
distributed. We also show that the vertex corrections to the electric
conductivity, which can be large when there is significant anisotropy in the
impurity scattering, become unimportant as the magnetic field is increased. For
the thermal conductivity, the corrections remain negligible as in the absence
of a magnetic field.
