  The thermoelectric Hall effect is the generation of a transverse heat current
upon applying an electric field in the presence of a magnetic field. Here we
demonstrate that the thermoelectric Hall conductivity $\alpha_{xy}$ in the
three-dimensional Dirac semimetal ZrTe$_5$ acquires a robust plateau in the
extreme quantum limit of magnetic field. The plateau value is independent of
the field strength, disorder strength, carrier concentration, or carrier sign.
We explain this plateau theoretically and show that it is a unique signature of
three-dimensional Dirac or Weyl electrons in the extreme quantum limit. We
further find that other thermoelectric coefficients, such as the thermopower
and Nernst coefficient, are greatly enhanced over their zero-field values even
at relatively low fields.
