Spontaneous spin polarization in quantum point contacts

We investigate ballistic transport in quantum point contacts fabricated from high mobility two-dimensional hole gas. At low temperatures conductance quantization in units of 2e2/h is observed. Incorporation of point contacts into a magnetic focusing device allows us to measure relative population of the spin subbands. While both subbands are found to be equally populated at the 2e2/h plateau, there is a finite polarization when conductance is < 2e2/h. Polarization is stronger in samples with well defined '0.7 structure'.