-Have we got any butter?
- No, not at all. (1)
- No, not in the least (2)
- No, not an ounce. (3)
Which of the above replies is grammatical?
Could suggest other possible replies expressing the same idea?
They're all grammatical, but the first two are inappropriate in the context. You use 'not at all' and 'not in the least' to counter someone's expectation of a negative:
'Do you mind if I open the window' [I'm worried in case you think I should not]
'No, not at all'/'No, not in the least'/'No, on the contrary'
Another way of saying 'No, not an ounce' is 'No, none at all'. There are also other nouns designating a small amount - 'scrape'/'smear'... Be careful which alternative noun you use though - 'No, not a crumb' would sound very strange in this context (although, over time, substance specific nouns can become generalized - the French negative particle rien is derived from the word for 'matter'/'thing'); indeed the Italian negative particle mica is related to the French for 'crumb' - miette. Something similar is happening to the English "jot" (the noun, not the verb) - Iota and Jot - Wikipedia, the free encyclopedia : the phrase 'not a jot' doesn't necessarily refer to writing.