  We investigate the carrier-envelope phase and intensity dependence of the
longitudinal momentum distribution of photoelectrons resulting from
above-threshold ionization of argon by few-cycle laser pulses. The intensity of
the pulses with a center wavelength of 750\,nm is varied in a range between
$0.7 \times 10^{14}$ and $\unit[5.5 \times 10^{14}]{W/cm^2}$. Our measurements
reveal a prominent maximum in the carrier-envelope phase-dependent asymmetry at
photoelectron energies of 2\,$U_\mathrm{P}$ ($U_\mathrm{P}$ being the
ponderomotive potential), that is persistent over the entire intensity range.
Further local maxima are observed at 0.3 and 0.8\,$U_\mathrm{P}$. The
experimental results are in good agreement with theoretical results obtained by
solving the three-dimensional time-dependent Schr\"{o}dinger equation (3D
TDSE). We show that for few-cycle pulses, the carrier-envelope phase-dependent
asymmetry amplitude provides a reliable measure for the peak intensity on
target. Moreover, the measured asymmetry amplitude exhibits an
intensity-dependent interference structure at low photoelectron energy, which
could be used to benchmark model potentials for complex atoms.
