Hi,
Peregrines do carry ducks regularly but I think that there are several factors influencing this behavior.
For example, we have the world's largest peregrine residing in Washington, the Peale's Falcon (incidentally, the type specimen was collected somewhere in south Puget Sound by Titian Ramsey Peale while he was a member of the Wilkes Expedition).
Female Peale's Falcons can weigh 1,350 grams (48 ounces) which is 3 pounds. That's a big peregrine. There are male Gyrfalcons that weigh in at 3 pounds or less.
Since they are so much bigger, Peale's are able to carry a larger prey item than the smaller, lighter Tundra Peregrine. Although I have to say that I once followed a tundra bird as it carried a Blue-winged Teal (very low to the sand) for over two miles on Padre Island, Texas. But I was also pushing her.
To complicate matters, male peregrines usually weigh about a third less than a female. So they are less capable of carrying the large duck species. They will take a few GW Teal, etc. but usually they seem to prefer smaller birds, like sandpipers which they can carry off and eat with no problem.
Most of the successful hunts that I have observed, primarily on the Samish Flats, have involved adult female peregrines and Bufflehead or GW Teal. A female peregrine can pack these quite nicely. These seem to be a preferred prey on Samish. The Wigeon kills are a different story. I have usually seen the falcons eating these heavier birds directly on the ground. They might be able to carry them, as Kelly mentions, but certainly not with a hungry Bald Eagle after them.
A falconer, Brett Gaussoin, reportedly observed an adult female peregrine carry a recently caught GW Teal up to a good elevation over the predator-dense Lummi Flats in winter and eat it entirely on the wing. This behavior was interpreted to be a response to avoid kleptoparasitism by other raptors on the ground.
I suspect that there are many other factors involved in carrying ducks. For example the level of hunger, motivation and personality of a specific bird, whether there is a strong wind, the appearance of competitors, the presence of strong updrafts at cliff nest sites, stuff like that.
I think that the main point, for me anyway, is that it is always noteworthy to see a successful peregrine hunt. In my experience, witnessing these events is relatively rare.

Bud Anderson
Falcon Research Group
Box 248
Bow, WA 98232
(360) 757-1911
bud at frg.org