What factors must a federal court consider to determine whether a defendant was a minor participant in a drug trafficking offence?

We review for clear error the district court's finding regarding the defendant's role in the offense. United States v. Flores-Payon, 942 F.2d 556, 560 (9th Cir.1991). The Sentencing Guidelines instruct the district court to decrease the offense level by two levels "[i]f the defendant was a minor participant in any criminal activity." U.S.S.G. 3B1.2(b). "A minor participant means any participant who is less culpable than most other participants, but whose role could not be described as minimal." U.S.S.G. 3B1.2, comment. (n. 3). To receive the adjustment, the defendant must be "substantially less culpable than the average participant." Id., comment. (backg'd); see United States v. Molina, 934 F.2d 1440, 1452 (9th Cir.1991) ("district court is not compelled to find that a defendant who is the least culpable in a particular transaction is therefore a minor participant"). The district court may consider such factors as whether the defendant was more than a one-time "mule" or "courier" of drugs, and whether he possessed a substantial amount of drugs. Flores-Payon, 942 F.2d at 561 (no clear error in denying adjustment where defendant participated in drug transaction); United States v. Sanchez-Lopez, 879 F.2d 541, 557 (9th Cir.1989) (same where defendant was driver of car containing "significant quantity" of heroin); cf. U.S.S.G. 3B1.2, comment. (n. 2) (downward adjustment for minimal participation appropriate "for someone who played no other role in a very large drug smuggling operation than to offload part of a single marihuana shipment, or in a case where an individual was recruited as a courier for a single smuggling transaction involving a small amount of drugs").