  The non-linear zero-differential resistance state (ZDRS) that occurs for
highly mobile two-dimensional electron systems in response to a dc bias in the
presence of a strong magnetic field applied perpendicular to the electron plane
is suppressed and disappears gradually as the magnetic field is tilted away
from the perpendicular at fixed filling factor $\nu$. Good agreement is found
with a model that considers the effect of the Zeeman splitting of Landau levels
enhanced by the in-plane component of the magnetic field.
