We have deposited ZnO:Al films with state-of-the-art electro-optical properties by reactive magnetron sputtering from a zinc target. Films with resistivities of 5X10(-4) Omega cm, good optical properties over the visible and near infrared, and with good uniformity are routinely produced. Resistivities down to 1.7X10(-4) Omega cm have been achieved but are accompanied by expected losses in optical properties. The fundamental mechanisms driving performance are largely determined by an interplay between microstructure and ionized impurity scattering. At the deposition level the parameters which control these mechanisms are substrate bombardment and substrate temperature. At substrate temperatures of 350 degrees C and sufficient source-to-substrate distances doping efficiencies approaching 100% can be achieved. At carrier concentrations of 4x10(20) cm(3) or higher there is a crossover from defect controlled transport to ionized impurity scattering dominated transport which constrains improvements in conductivity. This combined with free carrier effects on optical properties sets the limits for the electro-optical properties of these films. This understanding can be used to further develop the deployment of these films as transparent conductors.