Ballistic effusion of normal liquid ³He through nanoscale apertures

We have measured mass transport of normal liquid ³He through an array of submicron diameter apertures in a thin membrane. As the temperature is decreased we observe the crossover from viscous flow to ballistic effusion transport by quasiparticles. In this ballistic regime the quasiparticle mean free path is large compared to both the aperture diameter and the membrane thickness, and the flow conductance is temperature independent. At lowest temperatures, this experiment provides an analog of the electronic ballistic point contact for neutral Fermi liquids. The measured conductance is in quantitative agreement with theory.