Spin Properties of Low Density One-Dimensional Wires

We report conductance measurements of a ballistic one-dimensional (1D) wire defined in the lower two-dimensional electron gas of a GaAs/AlGaAs double quantum well. At low temperatures there is an additional structure at $0.7(2e^2/h)$ in the conductance, which tends to $e^2/h$ as the electron density is decreased. We find evidence for complete spin polarization in a weakly disorderd 1D wire at zero magnetic field through the observation of a conductance plateau at $e^2/h$, which strengthens in an in-plane magnetic field and disappears with increasing electron density. In all cases studied, with increasing temperature structure occurs at $0.6(2e^2/h)$. We suggest that the 0.7 structure is a many-body spin state excited out of, either the spin-polarized electron gas at low densities, or the spin-degenerate electron gas at high densities.