  We report measurements of the compressibility of a one-dimensional (1D)
quantum wire, defined in the upper well of a GaAs/AlGaAs double quantum well
heterostructure. A wire defined simultaneously in the lower well probes the
ability of the upper wire to screen the electric field from a biased surface
gate. The technique is sensitive enough to resolve spin-splitting of the
subbands in the presence of an in-plane magnetic field. We measure a
compressibility signal due to the 0.7 structure and study its evolution with
increasing temperature and magnetic field. We see no evidence of the formation
of the quasibound state predicted by the Kondo model, instead our data are
consistent with theories which predict that the 0.7 structure arises as a
result of spontaneous spin polarization.
