Well... Scale mail comes with gauntlets:
Scale Mail: This is a coat and leggings (and perhaps a separate skirt) of leather covered with overlapping pieces of metal, much like the scales of a fish. It includes gauntlets.
A breast plate comes with no gauntlets:
Breastplate: A breastplate covers the front and back. It comes with a helmet and matching greaves (plates to cover the lower legs). A light suit or skirt of studded leather beneath the breastplate protects limbs without restricting movement much.
Both weight the same amount. Both have the same arcane spell failure. (25%)
If anything, I'd expect scale mail to have a higher failure because it has gauntlets. But it doesn't. By that, we can deduce (woo Sherlock Holmes terms) that gauntlets don't add spell failure.
Now, the only place I can see this being contradicted is S&F partial armor rules, but I don't have the book at work.