In the following set of pairs, the first part of the pair is the standard negative form of contraction.
Are they ever heard of in conversations? If yes, are they as acceptable as the standard contracted forms?
He hasn't come yet. He's not come yet
He hasn't been working. He's not been working
They haven't come yet. They've not come yet
They haven't been working. They've not been working
Update: You can read more about less common contractions in the following posts: