Magnetotransport in GaAs/Ga1-xAlxAs heterostructures with a square array of
antidote has been investigated. Quantum lifetimes tau(q) are estimated fro
m the amplitudes of Shubnikov-de Haas oscillations and two values of tau(q)
are found at low and high magnetic fields. The critical field at which tau
(q) changes corresponds to the magnetic field where the cyclotron diameter
2R(c) is almost equal to the effective constriction length L-c between two
antidots. This suggests that the influence of scattering from antidots on t
au(q) is different in the regions where 2R(c) > L-c and where 2R(c) < L-c.
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