  Quantum reflection of thermal He atoms from various surfaces (glass slide,
GaAs wafer, flat and structured Cr) at grazing conditions is studied within the
elastic close-coupling formalism. Comparison with the experimental results of
B.S. Zhao et al, Phys. Rev. Lett. {\bf 105}, 133203 (2010) is quite reasonable
but the conclusions of the present theoretical analysis are different from
those discussed in the experimental work. The universal linear behavior
observed in the dependence of the reflection probability on the incident wave
vector component perpendicular to the surface is only valid at small values of
the component whereas, at larger values, deviation from the linearity is
evident, approaching a quadratic dependence at higher values. The surface
roughness seems to play no important role in this scattering. Moreover, the
claim that one observes a transition from quantum to classical reflection seems
to be imprecise.
