The Herm Sprenger ULTRA-PLUS Micro training collar is made with small dogs in mind.  Made with 1.5mm stainless steel, swivel added for ease of movemment.  Fits up to a 9.5" neck. Individual links can be added or removed for a better fit (sold separately.)

One one the most well known names in dog training.  Herm Sprenger products are extremely durable.  Most trainers recommend these for a safer alternative to choke collars.  If placed properly this collar will not apply pressure to the trachea.