  We construct an effective three dimensional theory for the MSSM at high
temperatures in the limit of large-$m_{A}$. We analyse the two-loop effective
potential of the 3D theory for the case of a light right handed stop to
determine the precise region in the $m_{h}$-$m_{\tilde{t}_{R}}$ plane for which
the sphaleron constraint for preservation of the baryon asymmetry is satisfied.
We also compare with results previously obtained usind 3D and 4D calculations
of the effective potential. A two-stage phase transition still persists for a
small range of values of $m_{\tilde{t}_{R}}$. The allowed region requires a
value of $m_{\tilde{t}_{R}} \lsi m_{t}$ and $m_{h} \lsi 100$ (110) GeV for
$m_{Q} = 300$ GeV (1 TeV).
