Electron transport through fully depleted ferroelectric tunnel barriers sandwiched between two metal electrodes and its dependence on ferroelectric polarization direction are investigated. The model assumes a polarization direction dependent ferroelectric barrier. The transport mechanisms, including direct tunneling, Fowler-Nordheim tunneling and thermionic injection, are considered in the calculation of the electroresistance as a function of ferroelectric barrier properties, given by the properties of the ferroelectric, the barrier thickness, and the metal properties, and in turn of the polarization direction. Large electroresistance is favored in thicker films for all three transport mechanisms but on the expense of current density. However, switching between two transport mechanisms, i.e., direct tunneling and Fowler-Nordheim tunneling, by polarization switching yields a large electroresistance. Furthermore, the most versatile playground in optimizing the device performance was found to be the electrode properties, especially screening length and band offset with the ferroelectric. ; Comment: 24pages, 7 figures, revised, one figure added