Spontaneous spin polarization in quantum point contacts.

We use spatial spin separation by a magnetic focusing technique to probe the polarization of quantum point contacts. The point contacts are fabricated from p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in the low-density regime, when the conductance of a point contact is tuned to < 2e2/h. Polarization is stronger in samples with a well-defined "0.7 structure."