  The incommensurate spin-density-wave (SDW) phase in (TMTTF)$_2$Br was
investigated through transport measurements under pressure and magnetic fields
parallel to the $c^{\ast}$ axis. For the incommensurate SDW phase of
(TMTTF)$_2$Br stabilized above 0.5 GPa, the SDW transition temperature $T_{\rm
SDW}$ increases with the applied magnetic field. The field dependence of
$T_{\rm SDW}$ is described by a quadratic behavior and the coefficient of the
quadratic term increases with increasing pressure. These results are consistent
with the prediction of the mean-field theory based on the suppression of the
SDW transition by two-dimensionality. From the relation between the coefficient
of the quadratic term and $T_{\rm SDW}$ at zero magnetic field, we determined
the role of electron correlation and two dimensionality in the SDW phase of
(TMTTF)$_2$Br under pressure and found that the SDW transition in (TMTTF)$_2$Br
can be well explained within the mean field theory by taking into account the
reduction of the coupling constant $N(0)I$ by pressure.
