Many-body spin interactions in semiconductor quantum wires

Zero length quantum wires (or point contacts) exhibit unexplained conductan ce structure close to 0.7 x 2e(2)/h in the absence of an applied magnetic f ield. We have studied the density- and temperature-dependent conductance of ultra-low-disorder GaAs/AlGaAs quantum wires with nominal lengths l = 0 an d 2 mum, fabricated from structures free of the disorder associated with mo dulation doping. In a direct comparision we observe structure near 0.7 x 2e (2)/h for l = 0, whereas the l = 2 mum wires show structure evolving with i ncreasing electron density to 0.5 x 2e(2)/h in zero magnetic field the valu e expected for an ideal spin-split sub-band. Our results suggest the domina nt mechanism through which electrons interact can be strongly affected by t he length of the 1D region.