IMO, Canon
SHOULD have supplied the 100-400mm f/4.5-5.6L IS ii lens with a foot that is grooved to accept Arca Compatible clamps but, they didn't! So you are stuck with using an accessory A/C clamp or purchasing an aftermarket foot. KIRK industries supplies a aftermarket foot that sells for $80 USD from B&H. I purchased one of these. Really Right Stuff also sells an aftermarket A/C foot at somewhere around the same price.
China has now joined the market and is supplying an aftermarket foot for $23.99 USD.
http://www.ebay.com/itm/Lens-Collar-...EAAOSwQaJXS~~p
I like the finger grip on this foot but I am not sure that I would want to trust the security of a lens that cost almost two thousand dollars to a no-name Chinese manufacturer.