  This article describes experiments performed with an ultracold gas of
metastable helium above the Bose-Einstein transition. The gas is trapped in an
harmonic magnetic potential and collective excitations are induced. Both the
frequency and the damping rate of the lowest monopole-quadrupole m = 0 mode of
excitation are studied at different temperatures and compared to theoretical
predictions. Results indicate that the gas goes from a collisionless regime
towards a hydrodynamic regime as the elastic collision rate increases, when one
goes down along the evaporative cooling ramp. However we find a discrepancy for
the collisional parameters in comparison with predictions relying on the value
of the scattering length previously estimated. Possible explanations are put
forward in the final discussion.
