Authentication with custom user model

I want to make login and registration for a custom user with only 5 fields: user name, name, password, linkedin id and mobile number.

I made registration successfully but I am stuck with login, I cannot authenticate my user. Is there any way to authenticate my user, or how can I login? Currently I am getting logged in by

user = Consultants.objects.get(Q(username= username) & Q(password= password))

But i want to make login by

user=authenticate(username=username,password=password)

Note:I don't want to use django default User Model For it. Please help me in this. Thanks in advance.

models.py

class Consultants(models.Model):
    first_name=models.CharField(max_length=255,blank=True,null=True)
    username=models.CharField(max_length=255,blank=True,null=True)
    password=models.CharField(max_length=50,blank=True,null=True)
    mobile_no=models.CharField(max_length=255,blank=True,null=True)
    linkedin_id=models.CharField(max_length=255,blank=True,null=True)
    is_active=models.BooleanField(default=False)

views.py

def register(request):
    context = RequestContext(request)
    registered = False
    print "inside register view"
    if request.method == 'POST':
        consultant_form = ConsultantsForm(data=request.POST)
        if consultant_form.is_valid():
            consultant = consultant_form.save(commit=False)
            consultant.save()
            registered = True
        else:
            print consultant_form.errors
    else:
         consultant_form = ConsultantsForm()
    return render_to_response(
            'register.html',
            {'consultant_form': consultant_form, 'registered': registered},
            context_instance=RequestContext(request))


def login_user(request):
    context = RequestContext(request)
    if request.method == 'POST':
        username = request.POST['username']
        password = request.POST['password']
        print type(username)
        try:
            user = Consultants.objects.get(Q(username= username) & Q(password= password))
            user = authenticate(username=username, password=password)
            if user.is_active:
                user.backend = 'django.contrib.auth.backends.ModelBackend'
                login(request, user)
                a= request.user.username
                return HttpResponse("welcome......you are succesfuly log in")
            else:
                return HttpResponse("Your  account is disabled.")
        except ObjectDoesNotExist:
            return HttpResponse("INvalid User")