Is a negative hiv antibody test taken 6 weeks after possible exposure conclusive?

Not necessarily.: Officially - the window period for HIV seroconversion (converting from a negative antibody to positive antibody test) can be up to 3 months. Most seroconversion should happen by 1 month but some can take up to three month. One option is to take a more sensitive test such as a HIV RNA (or viral load) test - this should pick up an infection after 2 weeks and would be conclusive at 6 weeks.

Depends on the test: If you had a 4th generation test (also called duo or combo test), it was conclusive -- any time more than 4 weeks after exposure. If a third generation antibody test, 6 weeks probably is conclusive, but official advice would be testing at 3+ months after exposure.