  From the Dirac equation of an electron in an anisotropic conduction band, the
anisotropy of its motion dramatically affects its interaction with applied
electric and magnetic fields. The quantum spin Hall effect (QSHE) is observable
in two-dimensional metals without spin-orbit coupling. The dimensionality of
the Zeeman interaction plays an important role in the QSHE, and profoundly
modifies many interpretations of measurements of the Knight shift and of the
upper critical field in highly anisotropic superconductors.
