Novel measurement techniques for probing quantum point contacts

Conductance measurements of quantum point contacts (QPCs) reveal an anomalous plateau at roughly 0.7 x 2e$^{2}$/h, when the mode occupation is just short of making a fully transmitting 1D channel available. Past experiments have built a consensus that this so-called ``0.7 structure'' is related to electron spin and electron-electron interaction, but the detailed description remains controversial. ~We have performed measurements on two new kinds of devices which give new insight into the interactions of electrons in these clean quasi-one dimensional systems. One device allows us to measure the compressibility of the electrons in a QPC for the first several conduction modes. ~Comparison with density functional calculations give new information about the relative importance of interactions (including exchange) as the density in the QPC is depleted.~ The second device allows us to measure the local density of states (DOS) in the QPC as we tunnel directly into the constriction.~ Deviations from the 1D DOS would help to develop a more complete picture of the transport through a QPC. We acknowledge support from the ONR Young Investigator Program, Award No. N00014-01-1-0569 and a Research Corporation Research Innovation Award, No. RI1260.