The transfer width is not large enough. The open channels have been resolved only partly.
He Scattering on one-dimensional corrugated model surface. Lattice constant 10 a.u. is appr. equal to  transfer width.
 

Remember for the pioneers of thermal energy scattering on solid surfaces (TEAS):

[1] I. Estermann, O. Stern Z. Physik vol. 61, 953 (1930).
[2] I. Estermann, R. Frish and O. Stern, Z. Physik vol. 73, 348 (1931)
[3] R. Frish and O. Stern, Z. Physik vol. 84, 430 (1930)
[4] T.H. Johnson, Phys. Rev. vol 35, 650 (1930); vol. 37, 847 (1931)

They demonstrated the wave property of He an H atomic beam in the case of scattering on solid surfaces LiF and NaCl.