  We present a numerical study of spin relaxation in a semiclassical electron
ensemble in a large ballistic quantum dot. The dot is defined in a GaAs/AlGaAs
heterojunction system with a two-dimensional electron gas, and relaxation
occurs due to Dresselhaus and Rashba spin orbit interaction. We find that
confinement in a micronscale dot can result in strongly enhanced relaxation
with respect to a free two-dimensional electron ensemble, contrary to the
established result that strong confinement or frequent momentum scattering
reduces relaxation. This effect occurs when the size of the system is on the
order of the spin precession length, but smaller than the mean free path.
