  We report on tunnel ionization of Xe by 2-cycle, intense, infrared laser
pulses and its dependence on carrier-envelope-phase (CEP). At low values of
optical field ($E$), the ionization yield is maximum for cos-like pulses with
the dependence becoming stronger for higher charge states. At higher
$E$-values, the CEP dependence either washes out or flips. A simple
phenomenological model is developed that predicts and confirms the observed
results. CEP effects are seen to persist for 8-cycle pulses. Unexpectedly,
electron rescattering plays an unimportant role in the observed CEP dependence.
Our results provide fresh perspectives in ultrafast, strong-field ionization
dynamics of multi-electron systems that lie at the core of attosecond science.
