If the interface behaves correctly as a Schottky barrier, the series resistance is that arising from the metal (which is negligible), from the semiiconductor bulk, from the connections. It can be evaluated by the slope of the IV curve in ON mode.
In general there are several things which can affect the series resistance such as transport properties of the bulk semiconductor and the height of barrier. Check the thermionic emission and the possible tramps which can affect the transport properties. Moreover, the length distance between the Schottky barrier and the Ohmic contact should be the low to avoid serial resistance. In addition, if your specific contact resistance of your Ohmic contact is high it is going to increase the series resistance of your diode. For more details, it is important to know what is your specific Schottky structure. What are your Metals and semiconductor?