(not) at all; (not) a bit; (not) in the least
Used with a negative predicate to mean 'not even a little'. Stronger than あまり～ない and closer to 全然～ない in everyday speech.
I don't understand what he's saying at all.
I'm not tired in the slightest.
He isn't the least bit sorry.
This dish isn't spicy at all.
少しも emphasizes 'not even a little' and is softer than 一切, which flatly denies any exception at all.
少しも emphasizes 'not even a little' in a straightforward way, whereas 何とも in negative sentences often implies 'nothing worth mentioning'.
少しも negates a small amount or degree ('not even a little'), whereas 何一つ negates a count of distinct items ('not a single thing').
少しも emphasizes 'not even a little' with a negative, whereas 決して stresses a firm denial or prohibition ('never, under any circumstances').
少しも is a more colloquial way to say '(not) at all' with negatives, while 何ら is more formal and emphatic.