  We report a theoretical calculation explaining the quantum Nernst effect
observed experimentally in a bismuth single crystal. Generalizing the
edge-current picture in two dimensions, we show that the peaks of the Nernst
coefficient survive in three dimensions due to a van Hove singularity. We also
evaluate the phonon-drag effect on the Nernst coefficient numerically. Our
result agrees with the experimental result for a bismuth single crystal.
