Many-body spin-related phenomena in ultra low-disorder quantum wires

Zero length quantum wires (or point contacts) exhibit unexplained conductance structure close to $0.7\times {2e}^2/h$ in the absence of an applied magnetic field. We have studied the density- and temperature-dependent conductance of ultra low-disorder $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wires with nominal lengths $l=0,$ 0.5, and 2 $\mu \mathrm{m},$ fabricated from structures free of the disorder associated with modulation doping. In a direct comparison in zero magnetic field we observe structure near $0.7\times {2e}^2/h$ for $l=0,$ whereas the $l=2\mathrm{}$ $\mu \mathrm{m}$ wires show structure evolving with increasing electron density to $0.5\times {2e}^2/h,$ the value expected for an ideal spin-split subband. For intermediate lengths $(l=0.5\mathrm{}$ $\mu \mathrm{m})$ the feature at $0.7\times {2e}^2/h$ evolves to $0.55\times {2e}^2/h$ with increasing density. Our results suggest the dominant mechanism through which electrons interact can be strongly affected by the length of the one-dimensional region.