Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots

The dependence of conductance fluctuations on parallel magnetic field is used as a probe of spin degeneracy in open GaAs quantum dots. The variance of fluctuations at high parallel field is reduced from the low-field variance (with broken time-reversal symmetry) by factors ranging from roughly 2 in a $1\mu \mathrm{m}{}^2$ dot to greater than 4 in $8\mu \mathrm{m}{}^2$ dots. The factor of 2 is expected for Zeeman splitting of spin-degenerate channels. A possible explanation for the larger suppression based on field-dependent spin-orbit scattering is proposed.