  For a broad range of electron densities $n$ and temperatures $T$, the
in-plane magnetoconductivity of the two-dimensional system of electrons in
silicon MOSFET's can be scaled onto a universal curve with a single parameter
$H_{\sigma}(n,T)$, where $H_{\sigma}$ obeys the empirical relation
$H_{\sigma}=A (n) [\Delta(n)^2 +T^2]^{1/2}$. The characteristic energy $k_B
\Delta$ associated with the magnetic field dependence of the conductivity
decreases with decreasing density, and extrapolates to 0 at a critical density
$n_0$, signaling the approach to a zero-temperature quantum phase transition.
We show that $H_{\sigma}=AT$ for densities near $n_0$.
