  The colliding between an ultra-intense laser pulse with a high energy
electron beam is not only an important source for high-brightness gamma-rays
but also a powerful approach to exploit new physics in the exotic strong-field
QED regime. In the cross-colliding geometry, when radiation-reaction (RR) force
is interpreted by the classical Landau-Lifshitz equation, we found that there
is a distinctive barrier that allows penetration of electrons at energies
beyond the barrier and blocks those of lower energies. While in the QED
perspective, electrons can be well reflected (transmit) in the regime where
complete transmission (reflection) is allowed classically. The reflection
(transmission) is guaranteed by the quantum nature of radiation but forbidden
by classical description. This effect is accompanied by the blurring of the
angular distribution for scattered electrons and becomes significant for laser
intensities at $2\times10^{23}\ \mathrm{W/cm^2}$ and electron energies of $\sim
10^2\ \mathrm{MeV}$; thus could be measured in the up-coming 10-100PW laser
facilities. By detecting the reflection rate of the energetic electron beam
after colliding and resolve the angular distribution, the results are capable
of identifying the boundaries between classical and QED approaches in the
strong field regime and testifying the various models describing the
fundamental process.
