  The flow of liquid helium through a single nanohole with radius smaller than
25 nm was studied for the first time. Mass flow was induced by applying a
pressure difference of up to 1.4 bar across a 50 nm thick Si3N4 membrane and
was measured directly by means of mass spectrometry. In liquid He I, we
experimentally show that the fluid is not clamped by the short pipe with
diameter-to-length ratio D/L~1, despite the small diameter of the nanohole.
This viscous flow is quantitatively understood by making use of a model of flow
in short pipes. In liquid He II, a two-fluid model for mass flow is used to
extract the superfluid velocity in the nanohole for different pressure heads at
temperatures close to the superfluid transition. These velocities compare well
to existing data for the critical superflow of liquid helium in other confined
systems.
