Possible Spin Polarization in a One-Dimensional Electron Gas.

In zero magnetic field, conductance measurements of clean one-dimensional (1D) constrictions defined in GaAs/AlGaAs heterostructures show up to 26 quantized ballistic plateaus, as well as a structure close to $0.7({2e}^{2}/h)$. In an in-plane magnetic field all the 1D subbands show linear Zeeman splitting, and in the wide channel limit the $g$ factor is $\ensuremath{\mid}g\ensuremath{\mid}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.4$, close to that of bulk GaAs. For the last subband, spin splitting originates from the structure at $0.7({2e}^{2}/h)$, indicating spin polarization at $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$. The measured enhancement of the $g$ factor as the subbands are depopulated suggests that the ``0.7 structure'' is induced by electron-electron interactions.