(Microsoft Word - Paper w/ fig\311)

Distributions of Coulomb blockade peak spacing are reported for large ensembles of both unbroken (magnetic field B = 0) and broken (B ≠ 0) time reversal symmetry in GaAs quantum dots. Both distributions are symmetric and roughly gaussian with a width ~ 2–6% of the average spacing, with broad, non-gaussian tails. The distribution is systematically wider at B = 0 by a factor of ~ 1.2 ± 0.1. No even-odd spacing correlations or bimodal structure in the spacing distribution is found, suggesting an absence of spin-degeneracy. There is no observed correlation between spacing and peak height.