  We investigate Lyapunov exponents of Brownian motion in a nonnegative
Poissonian potential $V$. The Lyapunov exponent depends on the potential $V$
and our interest lies in the decay rate of the Lyapunov exponent if the
potential $V$ tends to zero. In our model the random potential $V$ is generated
by locating at each point of a Poisson point process with intensity $\nu$ a
bounded compactly supported nonnegative function $W$. We show that for
sequences of potentials $V_n$ for which $\nu_n \|W_n\|_1 \sim D/n$ for some
constant $D > 0$ ($n \to \infty$), the decay rates to zero of the quenched and
annealed Lyapunov exponents coincide and equal $c n^{-1/2}$ where the constant
$c$ is computed explicitly. Further we are able to estimate the quenched
Lyapunov exponent norm from above by the corresponding norm for the averaged
potential.

Probability