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

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