  We experimentally study the energy gap within the incompressible strip at
local filling factor $\nu_c=1$ at the quantum Hall edge for samples of very
different mobilities. The obtained results indicate strong enhancement of the
energy gap in comparison to the single-particle Zeeman splitting. We identify
the measured gap as a mobility gap, so a pronounced experimental in-plane
magnetic field dependence can both be attributed to the spin effects as well as
to the change in the energy levels broadening.
