Inside the ADC circuitry there's a capacitor, which needs to be charged with different voltages when reading several channels. If there's no external capacitor, you should not limit the input by more than 10k, according to this hint
So the cap @jremington proposed rather helps to feed the ADC than to smoothen the input.
BTW: the discrepancy between the 100M input impedance and the 10k I mentioned is also addressed in the referencing link.