Not sure on the 16 series parts, but on the 18 and 24 series parts, you need to make sure things like BOR and WDT are disabled to get minimal consumption. Even so, the two or three micro-amps I usually have no trouble getting them down to has been plenty low enough for my battery powered applications. The XL versions available both in the 18 and 24 series are supposed to go into the nA. If you really need to get your power consumption down that low, you should look into using one of the 18 series XL parts. In general, 18 series parts are actually nicer to work with than the 16 series. My favorite is the 24 and companion DSP series. Those are brilliant parts IMO.