  The inversion layer of p-InSb(110) obtained by Cs adsorption of 1.8 % of a
monolayer is used to probe the Landau level wave functions within smooth
potential valleys by scanning tunnelling spectroscopy at 14 T. The nodal
structure becomes apparent as a double peak structure of each spin polarized
first Landau level, while the zeroth Landau level exhibits a single peak per
spin level only. The real space data show single rings of the valley-confined
drift states for the zeroth Landau level and double rings for the first Landau
level. The result is reproduced by a recursive Green's function algorithm using
the potential landscape obtained experimentally. We show that the result is
generic by comparing the local density of states from the Green's function
algorithm with results from a well controlled analytic model based on the
guiding center approach.
