  We consider second order elliptic operators with real, nonsymmetric
coefficient functions which are subject to mixed boundary conditions. The aim
of this paper is to provide uniform resolvent estimates for the realizations of
these operators on $L^p$ in a most direct way and under minimal regularity
assumptions on the domain. This is analogous to the main result in [Chill et
al. 2006]. Ultracontractivity of the associated semigroups is also considered.
All results are for two different form domains realizing mixed boundary
conditions. We further consider the case of Robin -- instead of classical
Neumann -- boundary conditions and also allow for operators inducing dynamic
boundary conditions. The results are complemented by an intrinsic
characterization of elements of the form domains inducing mixed boundary
conditions.
