Task breakdown of a user story is done at iteration planning by the whole team. Since all team members are present, they should have a fairly good understanding of the tasks. If not, the team is there to help them.

The person giving the estimate can use his personal experience to give an estimate based on a similar task he has done before. If the experience is not there, other team members can help by providing their input based on their own experience. Very often, pairs will form for specific tasks at this stage. The most important aspect to keep in mind is that the person responsible for the task is the one giving the estimate.