Current-voltage phase shift measurements are reported between electrod
es situated near the rim of an array located below a charged liquid he
lium surface. Data are taken in magnetic fields up to 20 T at a temper
ature near 2 K. Under these conditions, the ultra-quantum limit is rea
ched, where the Landau level width is larger than the electron thermal
energy. Interpreting the data in terms of the AC Hall effect, or edge
magnetoplasmon, suggests that the Hall resistivity deviates from its
classical value for fields greater than 10 T.