Let $\Gamma$ be a discrete group with a generating set $S$. Let $p_c(\Gamma,S)$ be the critical probability for percolation of the Cayley graph of $\Gamma$. Is it known that if $\Gamma$ is non-amenable then there exists a generating set $S$ such that $p_c(\Gamma,S)<\frac{1}{2}$ (or some other constant)?

The details on the question:

Let $0<p<1$ , the Bernoulli bond percolation of the Cayley graph defined as follows: the edge is open with probability $p$ and closed with $1-p$. The connected components of the graph are those that spanned by open edges. Let $\theta(p)$ be the probability that the origin belongs to an infinite open component. Then $p_c(\Gamma, S)=\sup (p:\theta(p)=0)$.