  We compute the nonequilibrium real-time evolution of an O(N)-symmetric scalar
quantum field theory from a systematic 1/N expansion of the 2PI effective
action to next-to-leading order, which includes scattering and memory effects.
In contrast to the standard 1/N expansion of the 1PI effective action, the
next-to-leading order expansion in presence of a possible expectation value for
the composite operator leads to a bounded time evolution where the truncation
error may be controlled by higher powers in 1/N. We present a detailed
comparison with the leading-order results and determine the range of validity
of standard mean field type approximations.
  We investigate ``quench'' and ``tsunami'' initial conditions frequently used
to mimic idealized far-from-equilibrium pion dynamics in the context of
heavy-ion collisions. For spatially homogeneous initial conditions we find
three generic regimes, characterized by an early-time exponential damping, a
parametrically slow (power-law) behavior at intermediate times, and a late-time
exponential approach to thermal equilibrium. The different time scales are
obtained from a numerical solution of the time-reversal invariant equations in
1+1 dimensions without further approximations. We discuss in detail the
out-of-equilibrium behavior of the nontrivial n-point correlation functions as
well as the evolution of a particle number distribution and inverse slope
parameter.
