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 ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ 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 ${0.6(2e}^2/h).\mathrm{}$ 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.