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Zero length quantum wires (or point contacts) exhibit unexplained conductance structure close to 0.7× 2 e 2/h in the absence of an applied magnetic field. We have studied the density-and temperature-dependent conductance of ultra low-disorder GaAs/Al x Ga 1− x As quantum wires with nominal lengths l= 0, 0.5, and 2 μ 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× 2 e 2/h for l= 0, whereas the l= 2 μ m wires show structure evolving with increasing electron density to 0.5× 2 e 2/h, the value expected for an ideal spin-split subband. For intermediate lengths (l= 0.5 μ m) the feature at 0.7× 2 e 2/h evolves to 0.55× 2 e 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.