Being sterilized or using any other means of contraception don't have anything to do with observing Iddah period.
In fact preventing Ikhtilate Miah* is one of the reasons of putting this law and not the entire reason.
It is said that the respect of the previous marriage is among the reasons.
Unless she has reached the age of menopause, so in this case there is no need for observing Iddah period.
*It means the mixture of semens and to avoid uncertainty in whose child the foetus will be, is intended by it.