  We monitor the Landau-Zener dynamics of a single-ion magnet in a
spin-transistor geometry. For increasing field-sweep rates, the spin reversal
probability shows increasing deviations from that of a closed system. In the
low-conductance limit, such deviations are shown to result from a dephasing
process. In particular, the observed behaviors are succesfully simulated by
means of an adiabatic master equation, with time averaged dephasing (Lindblad)
operators. The time average is tentatively interpeted in terms of the finite
time resolution of the continuous measurement.
