We prove a Plancherel theorem for a nonlinear Fourier transform in two dimensions arising in the Inverse-Scattering method for the defocusing Davey-Stewartson II equation. We then use it to prove global well-posedness and scattering in $L^2$ for defocusing DSII. This Plancherel theorem also implies global uniqueness in the inverse boundary value problem of Calderon in dimension $2$, for conductivities $\sigma>0$ with $\log \sigma \in \dot H^1$