  We analyzed the two-dimensional (2D) electron momentum distributions of
high-energy photoelectrons of atoms in an intense laser field using the
second-order strong field approximation (SFA2). The SFA2 accounts for the
rescattering of the returning electron with the target ion to first order and
its validity is established by comparing with results obtained by solving the
time-dependent Schr\"{o}dinger equation (TDSE) for short pulses. By analyzing
the SFA2 theory, we confirmed that the yield along the back rescattered ridge
(BRR) in the 2D momentum spectra can be interpreted as due to the elastic
scattering in the backward directions by the returning electron wave packet.
The characteristics of the extracted electron wave packets for different laser
parameters are analyzed, including their dependence on the laser intensity and
pulse duration. For long pulses we also studied the wave packets from the first
and the later returns.
