  We have measured the interaction between $^4$He gas at 4.2$~$K and a
high-quality nano-electro-mechanical string device for its first 3 symmetric
modes (resonating at 2.2$~$MHz, 6.7$~$MHz and 11$~$MHz with quality factor $Q >
0.1$ million) over almost 6 orders of magnitude in pressure. This fluid can be
viewed as the best experimental implementation of an almost-ideal monoatomic
and inert gas which properties are tabulated. The experiment ranges from high
pressure where the flow is of laminar Stokes-type presenting slippage, down to
very low pressures where the flow is molecular. In the molecular regime, when
the mean-free-path is of the order of the distance between the suspended
nano-mechanical probe and the bottom of the trench we resolve for the first
time the signature of the boundary (Knudsen) layer onto the measured
dissipation. Our results are discussed in the framework of the most recent
theories investigating boundary effects in fluids (both analytic approaches and
Monte-Carlo DSMC simulations).
