  We study a triangular frustrated antiferromagnetic Heisenberg model with
nearest-neighbor interaction $J_{1}$ and third-nearest-neighbor interactions
$J_{3}$ by means of Schwinger-boson mean-field theory. It is shown that an
incommensurate phase exists in a finite region in the parameter space for an
antiferromagnetic $J_{3}$ while $J_{1}$ can be either positive or negtaive. A
detailed solution is presented to disclose the main features of this
incommensurate phase. A gapless dispersion of quasiparticles leads to the
intrinsic $T^{2}$-law of specific heat. The local magnetization is
significantly reduced by quantum fluctuations (for S=1 case, a local
magnetization is estimated as $m=<S_{i}> \approx0.6223$). The magnetic
susceptibility is linear in temperature at low temperatures. We address
possible relevance of these results to the low-temperature properties of
NiGa$_{2}$S$_{4}$. From a careful analysis of the incommensurate spin wave
vector, the interaction parameters for NiGa$_{2}% $S$_{4}$ are estimated as,
$J_{1}\approx-3.8755$K and $J_{3}\approx14.0628$K, in order to account for the
experimental data.
