  In this paper we study spectral estimates of the $p$-Laplace Neumann operator
in conformal regular domains $\Omega\subset\mathbb R^2$. This study is based on
(weighted) Poincar\'e-Sobolev inequalities. The main technical tool is the
composition operators theory in relation with the Brennan's conjecture. We
prove that if the Brennan's conjecture holds then for any $p\in (4/3,2)$ and
$r\in (1,p/(2-p))$ the weighted $(r,p)$-Poincare-Sobolev inequality holds with
the constant depending on the conformal geometry of $\Omega$. As a consequence
we obtain classical Poincare-Sobolev inequalities and spectral estimates for
the first nontrivial eigenvalue of the $p$-Laplace Neumann operator for
conformal regular domains.
