  Coherent single electron spin oscillation in a double quantum dot system
driven by a magnetic electron spin resonance field is studied theoretically
using a Bloch-type rate equation approach. The oscillation frequency and
relaxation time obtained using typical model parameters are consistent with
experiment findings. The dominant decoherence mechanism is identified to be a
leakage current through a Coulomb blockade barrier at a quantum dot during the
spin manipulation. Nuclear field fluctuations which induce a long relaxation
time are found to contribute only negligibly to the decoherence despite an
earlier suggestion.
